A technique for forming a transistor by using a semiconductor over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. Silicon is known as a semiconductor applicable to a transistor.
In recent years, transistors including oxide semiconductors (typically, In—Ga—Zn oxide) have been actively developed. Oxide semiconductors have been researched since early times. In 1988, there was a disclosure of a crystal In—Ga—Zn oxide that can be used for a semiconductor element (see Patent Document 1).
Non-Patent Document 1 discloses a homologous series represented by In1−xGa1+xO3(ZnO)m (−1≤x≤1, and m is a natural number). Furthermore, Non-Patent Document 1 discloses a solid solution range of the homologous series. For example, in the solid solution range of the homologous series in the case where m is 1, x ranges from −0.33 to 0.08, and in the solid solution range of the homologous series in the case where m is 2, x ranges from −0.68 to 0.32.
Furthermore, a technique for forming a transistor using an In—Ga—Zn-based oxide semiconductor is disclosed (see, for example, Patent Document 2).
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. S63-239117    [Patent Document 2] Japanese Published Patent Application No. 2007-096055[Non-Patent Document]    [Non-Patent Document 1] M. Nakamura, N. Kimizuka, and T. Mohri, “The Phase Relations in the In2O3—Ga2ZnO4—ZnO System at 1350° C.,” J. Solid State Chem., 1991, Vol. 93, pp. 298-315.